Identifying cod ecotypes in Icelandic waters

There are mounting evidences of a more complex structure within Atlantic cod (Gadus morhua) stocks than previously assumed. The Icelandic stock consists of many spawning components, with genetically distinct ecotypes that exhibit different migratory behaviour, potentially various conditions and growth properties which leads to differences in age-at-maturity and might possess different vulnerability to fishing pressures. Failure to recognise or account for stock diversity is of great concern because it can lead to erosion of spawning components, depletion of the smaller or less productive population, loss of genetic diversity and overfishing. Therefore, a development of a method to discriminate between the ecotypes in an easy, accurate and cost-efficient way is of particular value for the management of the stock. The aim of this thesis to test whether otolith shape can be used for identifying ecotypes found within the Icelandic cod stock. This was conducted by developing a discriminant function based on otoliths from cod that had been tagged with Data Storage Tags, which until now remains the only definitive way to identify between ecotypes. The discriminant function was also validated by exploring the relationship between otolith shape and the Pantophysin genotypes that have been shown to differ in frequency between the ecotypes. The results were promising with great success in identifying ecotypes. Otolith shape assessment is, therefore, suggested as a method fisheries scientists can use when exploring the cod stock in Iceland. In this thesis we also explored the applicability of gluing broken otoliths back together for use in population discrimination studies, with success.Rannís, Rannsóknamiðstöð Íslands. Rannsóknasjóður Háskóla Íslands. Verkefnasjóður Sjávarútvegsins, Sjávarútvegsráðuneytið